1. Field of the Invention
The present invention relates in general to a circuit for detecting a pilot signal contained within a number of transmitted signals and, more specifically, is directed to a circuit for detecting a pilot signal to identify which one of several different AM stereo transmitting systems is being received.
2. Description of the Prior Art
Commercial broadcast of amplitude-modulated (AM) stereo signals have only recently been made available to the public. While such AM stereo broadcasts are desirable and have been eagerly awaited by the public, nevertheless, no one particular system has been designated by the Federal Communications Commission as the standard and at present there are five different stereo systems vying for supremacy. The five different modulation systems used in transmitting AM stereo signals are: amplitude modulation-phase modulation (AM-PM), in which a carrier wave is amplitude-modulated (AM) by a sum signal (L+R) of a stereo left channel signal (L) and right channel signal (R) and the carrier wave is then phase modulated (PM) by a difference signal (L-R) from the same left and right stereo channels; amplitude modulation-frequency modulation (AM-FM), in which a carrier wave is amplitude modulated by the sum signal (L+R) and the carrier is also frequency-modulated by the difference signal (L-R); compatible-quadrature modulation (C-QUAM), in which a phase modulated signal, provided by balanced-modulating and adding (orthogonally modulating) two carrier waves of the same frequency but with a mutual phase difference of 90.degree., by the left channel signal (L) and the right channel signal (R) is amplitude-modulated by the sum signal (L+R); variable angle compatible phase multiplex (VCPM), which is an orthogonal modulation system but in which the phase-angle difference is controlled in response to the magnitude of the difference signal (L-R); and independent side band (ISB), in which by means of a 90.degree. phase-shifting circuit the carrier wave is amplitude-modulated by the sum signal (L+R) and is phase-modulated by the difference signal (L-R).
All of these five different AM stereo broadcasting systems are in use today and, thus, AM stereo receivers have been proposed that are capable of receiving and decoding or demodulating signals from all of these various AM stereo systems. Such receivers then must have the appropriate circuit construction that is then operably changed in response to the specific AM stereo signals being received at that time. Such change can be determined, and in fact controlled, by a pilot signal contained within the actual AM stereo signals that have been transmitted. Thus, it is necessary to provide a receiver having a circuit to detect a pilot signal that can be transmitted relative to any of the five different AM stereo signal formats.
One system that has been proposed to detect such different pilot signals employs a separate low-pass filter or bandpass filter for each of the various AM stereo broadcast systems, thereby detecting the pilot signals in an analog fashion. However, in order to provide such analog filter circuits capable of detecting all of these various pilot signals the circuit arrangement therefor becomes unduly complicated and complex and, moreover, the performance of such filter circuits is questionable and uncertain due to variations in the shape of the filter transfer characteristic, as well as the actual filtering capabilities which are a function of the manufacturing techniques employed in arranging the specific circuit elements in the filter circuit. Also proposed is a system for extracting the respective pilot signals that employs a separate phase-lock loop and attendant oscillator for each of the various different AM stereo broadcast systems. Further, it is possible in AM broadcasting that a certain phase modulated component, caused by less than accurate transmission at the AM broadcast station, can accompany the AM signals being broadcast, and this causes a misoperation or apparent malfunction at the receiver. Additionally, the circuits to detect the pilot signals are critical to system operation and noise and the like that are mixed with the transmitted signal around the frequency band of the pilot signal will result in faulty detection of such pilot signals and misoperation of the receiver.